A Use of Limit Cycle Oscillations to Obtain Robust Analog-to-Digital Converters

High quality analog-to-digital conversions are obtained using simple and inexpensive circuits that require no high-precision components. Samples of the analog signal are cycled rapidly through a coarse quantizer while the roundoff error is fed back and subtracted from the input. By means of this feedback, the coarse quantizations are caused to oscillate between levels, keeping their running average representative of the input. A binary coding of the quantized values, summed over Nyquist intervals, provides a high resolution PCM output. The precision is determined by a product of the cycle rate and the spacing of the coarse quantization levels. The system is surprisingly tolerant of inaccuracies in gains and threshold settings; indeed, it has many of the desirable properties of classical feedback servomechanisms. An 8-bit limit cycling converter intended for 1-MHz signal bandwidths has been fabricated of standard components that, in total, cost less than $150.